Bull trout concealment experimental data

Bull trout (Salvelinus confluentus) are challenging to detect as a result of the species cryptic behavior and coloration, relatively low densities in complex habitats, and affinity for cold, high clarity, low conductivity waters. Bull trout are also closely associated with the stream bed, frequently conceal in substrate, and this concealment behavior is poorly understood. Consequently, population assessments are problematic and biologists and managers often lack quantitative information to accurately describe bull trout distributions, estimate abundance, and assess status and trends; particularly for stream-dwelling populations. During controlled laboratory trials, we recorded concealment, resting, and swimming behavior of juvenile wild bull trout in response to: (1) constant and fluctuating water temperature, (2) presence or absence of light, and (3) substrate size. Light level had the strongest influence on wild fish concealment and more fish concealed as light levels increased from darkness to daylight. Wild fish were 14.5 times less likely to conceal in constant darkness and 4.1 times more likely to conceal in 12 h light x 12 h darkness compared to constant light. Wild fish were 6.2 times less likely to conceal in small (26-51 mm) substrate compared to larger (52-102 mm) substrate. As water temperature increased, fewer wild fish concealed. Knowledge of wild bull trout concealment will improve field sampling protocols and increase detection efficiencies. These data also enhance knowledge of bull trout niche requirements which informs conservation and restoration efforts.

溪红点鲑（Bull trout, Salvelinus confluentus）因兼具隐秘的行为与体色特征、复杂生境中种群密度相对偏低，且偏好寒冷、高透明度、低电导率的水域，因而难以被监测发现。此外，该物种常与河床紧密关联，频繁隐匿于底质之中，目前学界对这类隐匿行为的认知仍较为匮乏。
因此，其种群评估工作面临诸多挑战，生物学家与资源管理者往往缺乏定量数据，无法精准描述溪红点鲑的分布范围、估算种群丰度、评估其种群现状与变化趋势——对于溪流栖息种群而言，这一问题尤为突出。
在可控实验室试验中，我们记录了野生幼年溪红点鲑在以下三种条件下的行为响应：(1) 恒定水温与波动水温；(2) 有光照与无光照；(3) 底质粒径。
光照强度对野生溪红点鲑的隐匿行为影响最为显著：随着光照水平从黑暗逐步提升至日间光照，选择隐匿的个体数量逐渐增多。与持续光照条件相比，野生溪红点鲑在持续黑暗环境中的隐匿概率仅为前者的1/14.5；而在12小时光照/12小时黑暗交替的光周期条件下，其隐匿概率是持续光照下的4.1倍。相较于粒径为52-102 mm的大型底质，野生溪红点鲑在26-51 mm的小型底质中的隐匿概率仅为前者的1/6.2。随着水温升高，选择隐匿的野生溪红点鲑个体数量逐步减少。
掌握野生溪红点鲑的隐匿行为规律，将有助于优化野外采样方案、提升种群检测效率。本研究数据还能深化对溪红点鲑生态位需求的认知，为其保护与栖息地修复工作提供科学依据。